Variably Saturated Flow and Transport: Sorbing Solute

With variably saturated flow, fluids fill only part of the pore space. Flow properties depend on degree of saturation, making Richards’ equation nonlinear Often researchers use analytic expressions (e.g., van Genuchten or Brooks & Corey) to describe how material properties vary with the solution. This example also shows how to incorporate experimental data directly into the COMSOL Multiphysics model. Example based upon Hydrus2d Manual (Simunek and Van Genuchten, 1992) Variably saturated flow

Fluid in column moves into “disc” where it is distributed over given radius. Fluid moves from disc into dry soil. With good control on fluid (and contaminant) coming from disc, researchers analyze Subsurface properties and behaviors. Image from Department of Agriculture and Soil Science University of Sidney, Australia Disc permeameter

Axisymmetry 1.3 m Ground surface Inlet “just ponding” at known water height 2-layer soil column initially unsaturated to depth of about 1.2 m Extremely low permeability Upper soil layer Lower soil layer Problem set up

H p = pressure head Se= effective saturation S= specific storage C= specific moisture capacity K= hydraulic conductivity D= elevation  = fluid volume fraction (constitutive relation) Hp=0 , Se, C, K - Hp + Variably saturated flow equation Hp subscripts to denote dependency on Hp  NONLINEAR

We can set up the permeability and retention formulae three ways: (1)Using analytic formulae predefined from van Genuchten or Brooks & Corey (2)Defining your own expressions (3)By interpolating between experimental data Variably saturated flow equation

k r Relative permeability C Specific capacity Se Effective saturation  Volume liquid fraction (1) … van Genuchten (shown in “Sorbing Solute” model from ES Library)

k r Relative permeability C Specific capacity Se Effective saturation  Volume liquid fraction (3) … Interpolation from experimental data (shown in “Interpolation” model from ES Library)

Hp = 0 Hp = Hp(x,z,0) Axisymmetry No flow Leaky Specified pressure head Flow: Boundary and Initial Conditions

1 day5 days 10 days Day 5: Soil wetting up, still dry at surface far from disc Day 10: Almost all pore space filled with water. Day 1: Mostly unsaturated (Hp<0) Notice wetting front Flow Snapshots

Flow Movie

c = concentration  = liquid volume fraction  b = bulk density k p = linear sorption coefficient C = specific moisture capacity Hp= pressure head D = dispersion tensor q = specific discharge  = decay rate Variably saturated solute transport liquid concentration c solid concentration c p =  b k p c

c (r,z,0)=0 Transport: Boundary and Initial Conditions Axisymmetry Free advection No flux Specified concentration c=1 Free advection Initially pristine

Transport Concentration Snapshots Surface is liquid concentration; contours are pressure head 1 day 5 days

Transport Retardation Factor Sorption slows contaminant movement relative to water u solute =u water /RF Retardation greatest where pore space is emptiest

Transport Concentration Movie